1. Field of the Invention
This invention relates to a method and apparatus for providing cathodic protection to underground metallic structures using galvanic anodes, and more particularly relates to a casing for holding the galvanic anodes, a method of placing the casing near the underground metallic structure and replacing galvanic anodes.
2. Background Information
Cathodic protection of underground metallic structures is a method of preventing corrosion. Corrosion is a natural process that occurs when a refined metal wants to revert to the ore from which it was derived. For example, iron is not found in its refined state in nature. Therefore, an underground iron or steel pipe is essentially in an unstable state and can be expected to eventually revert to iron ore (corroding).
It is known that corrosion occurs where electric current flows away from the underground metallic structure into the electrolyte. This area is called the anodic area. Conversely, corrosion does not occur at the areas where electric current flows from the electrolyte onto the metallic structure. This is called the cathodic area. Cathodic protection involves providing current to the metallic structure with sufficient magnitude to polarize all the cathodic areas up to the open circuit potential of the anodic areas.
There are various methods of applying cathodic protection. One method is by using an impressed current system. This involves placing an anode in the electrolyte and connecting its positive terminal to a DC power source. The structure to be protected is then connected to the negative terminal of the power source. Because the power source is almost always a rectifier unit, this system is sometimes referred to as a rectifier type system.
An example of an impressed current system is U.S. Pat. No 3,725,669. The apparatus consists of a plastic casing having an upper imperforate section and a lower pipe section with a plurality of openings. The bottom of the plastic casing has a check valve. After lowering the pipe into the bore hole, a wash pipe is threadedly connected to the check valve so that the wash pipe extends through the casing. The end of the hose is connected to a source of clean water which pumps clean water into the hole. This water is pumped in until all of the mud has been cleared from the casing. The hose is then disconnected from the water supply and connected to a hopper containing a carbonaceous slurry material. This carbonaceous slurry is introduced into the area between the bore hole and casing. After this, a plurality of anodes (mounted on a support line) are lowered into the casing. The anodes are connected by electrical conduits to a rectifier unit. When the anodes need to be replaced, the rectifier unit is disconnected, the cap is removed and a hose carrying water (FIGS. 8 and 9) is placed into the casing to fluidize the carbonaceous material. After the carbonaceous material has been fluidized, the anodes are removed, and fresh anodes are placed inside the casing.
The impressed current system, however, is not practical for cathodically protecting underground metallic structures located in the vicinity of other infrastructures. This is because the impressed current system creates electrical interference and can cause detrimental corrosion effects on other metallic underground structures, such as water supply mains, underground electrical and telephone cables and manholes that are in the vicinity of impressed current systems. Thus, impressed current systems are generally found in rural areas where interference is less of a problem.
Additional problems with impressed current systems include the higher cost and extensive installation and maintenance required for these systems. Impressed current systems require electric meters, rectifier cover enclosures, ampere monitoring gauges, electrical breaker switches and lightning arrestors. These components add to the cost of producing, installing and maintaining these systems. Finally, the large rectifier units, being placed above the ground, would further congest already crowded urban pedestrian sidewalks and are unsightly.
Another method of cathodic protection involves using a galvanic anode. In this system, anodes made of materials such as magnesium and zinc are buried in the ground near the metallic structure to be protected. The anodes are then connected to a metallic structure to be protected. Because of the potential differences between the anode and the metallic structure to be protected, current flows from the galvanic anode to the metallic structure to be protected. See U.S. Pat. Nos. 2,527,361; 4,511,444; and 4,623,435.
In order to install a galvanic anode near an underground metallic structure, an area near the structure must be excavated and the galvanic anode buried in the soil. This method is costly and time consuming, especially if installation is to be done in urban areas, extensively paved areas and areas that may accommodate other underground utilities.
The cur cent flow from the galvanic anode provides protective current to the structure until the anode material has been consumed to the point that the amount of current supplied is no longer of practical value. Once consumed, installation of a new anode is required. Because of this, the remains of the spent galvanic anodes must be extracted from the ground and new galvanic anodes put in their place. This involves re-excavating the area where the exhausted anode is placed and placing a new anode in its place and refilling the excavation site. See U.S. Pat. No. 3,186,931. In certain areas, this process often involves tearing up pavement and subsequently restoring the excavation site. Permits and other government approvals must be obtained to perform this work. As will be appreciated, this is a labor intensive task that not only is expensive, but also causes disruption and inconvenience in the excavation area.
Despite the known apparatus and methods, there still remains a need for an apparatus and method of providing cathodic protection to underground metallic structures using galvanic anodes that is simple and effective and requires less time and disruption.